You're right. RT6 is most definitely an API SM oil. I might buy their argument if the WRX had a new engine, but it doesn't. IMO, they cannot PROVE that it was the oil especially since the original manual lists thicker viscosities. It also cannot be proven that you received the addendum.
$110 just for the oil change or does that include a diagnostics charge? Find another dealer and also bring your own oil from Wal-Mart.